The present invention relates to gas chromatograph mass spectrometry.
A gas chromatograph mass spectrometer (GC/MS) which is a combination of a gas chromatograph and a mass spectrometer is widely applied as an all-purpose analyzer which combines the high separation ability of the gas chromatograph and the excellent qualitative ability of the mass spectrometer. In the GC/MS, a mixture sample of an organic compound is separated by the gas chromatograph; elution gas of the gas chromatograph is guided to the mass spectrometer; and the mass spectrometer defines the component and determination.
For measuring the compound with the GC/MS, He (helium) and so on are used as the carrier gas of the gas chromatograph. The component which is separated at the column and eluted is ionized by an electron-impact ionization method (EI method) and so on, and the mass of the component is detected after being separated by the mass spectrometer. Accordingly, a mass chromatogram, a total ion chromatogram, or a mass spectrum is obtained. Thereafter, the determination can be conducted from a peak area and so on of a chromatogram, or the component can be determined by retention time, through a well-known method in the field of gas chromatography. Also, the component of the mass spectrum can be determined using a data base such as the NIST or the Wiley through a well-known method in the field of mass analysis.
Usually, in the field of gas chromatography, identification of an unknown component is conducted as follows. A standard sample of the measurement object component which is predicted to be contained is measured under a completely identical condition with the measurement of the unknown component, and identified by agreeing with the respective retention time. However, since the standard sample of each component has to be provided, dozens of standard samples have to be provided when many components are determined, and there are some substances which are difficult to obtain.
Also, the retention time in the gas chromatography differs from a wave length by a spectral analysis or a mass number by the mass analysis. The retention time is not the value which is unambiguously predicted based on properties of matter of the component substance, but is determined by many factors such as the type or size of the column, the temperature, the type, or pressure and flow of the carrier gas, and also the difference of an apparatus and so on. Therefore, even if an identical analysis condition is established, since a model of the apparatus, or conditions of the room temperature or the column and so on differs, there are no completely identical conditions. As a result, the retention time sometimes differs. In other words, even if the identical condition is established, not only when the flow or temperature is changed, but also when the apparatus or column is exchanged, or long periods have passed, the retention time changes. As a result, the standard sample of the measurement object component is required to be measured and the retention time is required to be re-measured.
Thus, conventionally, in order to eliminate the above-mentioned variable factors as much as possible, a retention index is used for the identification of the component. The retention index is an index which indexes the retention time of each component substance by the retention time of the predetermined standard substance (generally, n-alkane), and is unaffected by the difference of a GC condition, a column manufacturer, length, inside diameter, or film thickness and so on. Therefore, without having to measure the standard sample of the measurement object component, if the retention index of the measurement object component is determined, the component can be identified by obtaining and comparing the retention index of the unknown component. As a result, the standard sample is not required to be measured. (For example, see Journal of Chromatography, 91 (1074) 89-103.)
In addition to the EI method, an ionization method in mass analysis is a method in which reaction gas (methane, isobutene, ammonia, etc.) is introduced into an ionization chamber, and the reaction gas is ionized by an electron impact and so on. At the same time, the ionization method forms a negative ion by electron capture of an emitted electron. As described above, the chemical ionization such that the negative ion is formed is known as the Negative Chemical Ionization (NCI) method.
Since the electron is selectively introduced into the substance with high electron affinity in the NCI method, the compound with high electron affinity is selectively negatively-ionized. As a result, the influence of a sandwiched incidental material can be reduced, and sometimes, sensitivity to a certain type of compound can be improved approximately 10-100 times compared with the EI method which is usually used. The NCI method is used for the determination of a minor constituent in a variety of areas because of high selectability or high sensitivity as described above. However, capabilities of the identification and determination of the component by a mass spectrum of the NCI method is considered to be inferior to the EI method.
Ionization by the NCI method is known as an ionization method which is suitable for selectively ionizing a halogen compound and so on with a high electron affinity and measuring with high sensitivity. However, compared to the mass spectrum in the EI method, the mass spectrum in the NCI method has fewer fragment ions and the spectral pattern is easier. Also, depending on the condition at the time of the ionization, the pattern of the mass spectrum sometimes differs slightly, and the identification ability by the mass spectrum is considered to be inferior to that of the mass spectrum in the EI method. Therefore, in order to enhance the reliability of the identification, the confirmation of the agreement of the retention time which is a conventional technology is effective.
However, as mentioned above, in order to confirm the agreement of the retention time, the standard sample of the measurement object component which is predicted to be contained is measured, and the agreement of the retention time of the chromatogram is confirmed. Accordingly, the standard sample has to be provided for each component to be measured, so that when multiple components are analyzed, the variety is enormous, and sometimes it is difficult to obtain, or it can be very expensive. In addition, since the retention time changes depending on the model of the apparatus or by cutting the column, the standard sample is frequently required to be measured.
Furthermore, the method described in conventional technology in which the standard sample is not measured and the retention index is used is not available when the measurement is performed by the NCI method in the GC/MS. Generally, the retention index is defined by the retention time of the n-alkane; however, when the measurement is performed by the NCI method, the n-alkane is not detected, and the retention time of the n-alkane under the same condition with an unknown sample is not known. Therefore, the retention index of the unknown sample cannot be obtained, so that the identification of the chromatogram by the retention index without the analysis of the standard sample is impossible.
The present invention was developed in order to solve the above-described problems. An object of the invention is to frequently provide the standard sample of a measurement object component by a “retention index for the NCI” which can be used when a GC/MS analysis is conducted using the NCI method, and to conduct a determination of the component with high reliability by retention time information without any analysis.
Further objects and advantages of the invention will be apparent from the following description of the invention.